A well-known portable hand-held chain saw (called as “portable chain saw” hereinafter) includes a power unit having a handle and a driven component which is mounted on the power unit and provided with a saw chain and a chain support that supports the saw chain.
The power unit is typically provided with an engine, a fuel tank, an oil tank, a power transmission and so forth in a compact configuration, and equipped with a handle for holding and maneuvering the chain saw with both hands.
For the portable chain saw operated while being held with the handles, there is a demand for more compact power units for higher maneuverability.
The power unit includes an air-cooled engine which is cooled by air generated by blades of air-cooling fan provided to a flywheel, guided through an air-guiding passage in a spiral manner so as to enclose part of the external periphery of the flywheel that rotates in accordance with the engine, and emitted from an open port of the air-guiding passage towards a cylinder of the engine.
At a top of the cylinder, a spark plug is mounted facing the internal combustion chamber inside the cylinder and is connected to one end of a high-voltage cable. The other end of the high-voltage cable is connected to an ignition coil.
The ignition coil is disposed on a periphery of the flywheel. As a permanent magnet disposed on the flywheel passes in front of the ignition coil, the electromotive force is generated in such a way that the ignition is synchronized with the rotation of the crankshaft.
In the power unit, the ignition coil is desired to be arranged so as not to degrade the air-guiding efficiency.
If the ignition coil is attached on the surface of the cylinder above the open port of the air-guiding passage, the air flow from the open port is obstructed by the ignition coil before reaching the cylinder, leading to insufficient cooling of the cylinder. Moreover, the ignition coil provided directly to the cylinder that is a heat source of the engine is subject to heat damage, degrading the ignition reliability.
On the other hand, if the ignition coil is arranged to the periphery of the flywheel so as not to affect the air flow, the cap of the oil tank inlet or the like should be placed away from the flywheel so as to ensure a space for the ignition coil. As a result, the compactness required for highly maneuverable power unit will be sacrificed.